An Nd—Fe—B series sintered magnet is used as a high-performance permanent magnet for various devices, especially hard disk drives or various kinds of motors.
A remanence of an Nd—Fe—B series sintered magnet sometimes decreases (demagnetization) when the magnet is exposed to high temperature or a demagnetizing field is applied. Such demagnetization includes “reversible demagnetization” in which the remanence recovers when the temperature is returned to an ordinary temperature and “irreversible demagnetization” in which the remanence does not recover. Since the various devices mentioned above are used at various temperatures, there is a need that irreversible demagnetization does not occur even when a demagnetizing field is applied at a high temperature. A conventionally utilized technique is to substitute Nd in a main phase of an Nd—Fe—B series sintered magnet with a heavy rare-earth element such as dysprosium (Dy) by adding the heavy rare-earth element to the magnet so as to prepare a magnet having a high coercivity, in which irreversible demagnetization does not occur even when a demagnetizing field is applied at a high temperature. The “irreversible demagnetization” will be hereinafter expressed simply as “demagnetization” in the present specification.
In the method of substituting Nd in an Nd—Fe—B series sintered magnet with a heavy rare-earth element, however, the remanence still lowers when a coercivity increases. Moreover, it is required to reduce the amount of a heavy rare-earth element to be used while maintaining the heat resistance, since a heavy rare-earth element is a scarce resource. Therefore, a technique has been proposed to supply a heavy rare-earth element such as dysprosium from a diffusion source to the surface of a magnet and make a layer having a high density of a heavy rare-earth element at an outer periphery of the main phase of the magnet by thermal diffusion mainly consisting of grain boundary diffusion (WO 2007/102391). Disclosed in WO 2006/43348 is a technique to diffuse a heavy rare-earth element such as dysprosium from the surface of a magnet into the magnet. This enables manufacturing of a high-performance permanent magnet having an enhanced coercivity while inhibiting lowering of the remanence of the entire permanent magnet.
When a magnet is used for a motor or the like, it is important to know the magnetic force characteristic of the magnet, especially a demagnetizing factor due to heat or the like, in order to design a magnet appropriately and to decide specifications such as characteristics of a motor. Disclosed in Japanese Patent Application Laid-Open No. 2004-127056, Japanese Patent Application Laid-Open No. 2004-257879 and WO 2008/123251 are methods for evaluating demagnetization of a permanent magnet.